A device was constructed for the purpose of determining visual receptive fields using the Alopex principal, which was first described by Harth and Tzanakou (Vision Research 14, 1475-1482, 1974). In the current work the device and the method will be further tested, using microelectrode recordings from single fibers in frog tectum. BIBLIOGRAPHIC REFERSNCES: E. Harth. 1975. A Dynamic Theory of Perception. Symposium on Biocybernetics and Theoretical Neurobiology. 3rd European Meeting on Cybernetics & Systems Research, Vienna 1976 (Abstract enclosed). E. Harth. 1975. Visual Perception: A Dynamic Theory. Submitted to Biol. Cyber. (manuscripts enclosed).